Devices employed for removing dissolved ions from liquid using electrical fields include electrodialysis and electrodeionization devices used for such purposes as desalination of saltwater and removal of ionic contaminants from base solutions. A typical electrodialysis cell consists of a series of diluate compartments and a concentrate compartments sequentially formed between anion exchange membranes and cation exchange membranes placed between two electrodes. In almost all practical electrodialysis processes, multiple electrodialysis paired compartments made of alternating anion and cation exchange membranes are arranged into a configuration called an electrodialysis stack. Thus, an “electrodialysis cell” generally includes the combination of an electrodialysis stack, a pair of electrodes, and input and output fluid flow channels/passages. In such devices, these stacks are placed in the path of ions moving under the influence of an electric field, resulting in formation of alternating diluate and concentrate compartments. Ions are depleted from the diluate compartments and accumulated in the concentrate compartments, as is known it the art. In addition, specific spacers are typically incorporated in various forms between adjacent ion exchange membranes. This is done in order to facilitate the independent flow of the liquids in the diluate and concentrate compartments, as well as to create volume within each compartment, prevent leakage from the stack to the outside, and to maintain separation between adjacent anion and cation exchange membranes. Typically, input solutions are directed through specific flow channels positioned in the supporting endplates of the device, which in combination with flow passages in the spacers and membranes, enable the independent flows in the concentrate and diluate compartments.
Electrodeionization devices are generally distinguished from electrodialysis devices in that the space in the center of the cell, between the ion-selective membranes, is filled with a thin bed of ion-exchange resins in the diluate compartment, or in both the diluate and concentrate compartments. Electrodeionization devices are typically used for production of higher purity products. The membranes are separated from one another by a screen separator, and the ion exchange resins facilitate ion flow in the sparingly conductive high purity deionized products. While technically different, as used herein the terms “electrodialysis” and “electrodeionization” can be used interchangeably, unless otherwise stated.
The most common electrodialysis equipment uses conventional metallic electrodes and establishes an electric field through electrode reactions with the solutions placed adjacent to them. The electrodes used can also be of the capacitive type, which are capable of absorbing large amount of ions and capacitively establishing the electric field. U.S. Pat. No. 8,715,477 to Yazdanbod (the inventor of the present invention), which is incorporated herein by reference in its entirety, specifically teaches electric double layer capacitors, behavior of high electric capacity electrodes in confined containers, use of high electric capacity electrodes as means of capacitive generation of electric fields, and polarity reversals as means of avoiding electrode reactions. Experimental evidence and test results establishing the formation, voltage distribution, and operating conditions of Electric Double Layer Capacitors (EDLCs) is an important feature incorporated in the present invention.
Although the primary goal in using electrodialysis devices is to move the dissolved ions from the diluate compartments to the concentrate compartments, typically some water movement also occurs, thus reducing the volume of the desired product (which is the purified liquid in the diluate compartment). This reduces the efficiency of such devices in producing purified liquids, such as desalinated or deionized water. Movement of water molecules from the diluate compartments to the concentrate compartments mainly occurs by three processes: (1) movement of water molecules attached to individual ions as hydration water; (2) movement of water from within the pore structure of the ion exchange membranes by electro-osmosis; and (3) water movement by osmosis. In electrodialysis equipment, all of these water transfer processes occur simultaneously through the membranes defining the boundaries of the compartments.
First, as ions moving in such liquids are hydrated ions in which a number of water molecules are attached to individual ions, the movement of these ions from the diluate compartments to concentrate compartments also results in the transfer of these attached water molecules. This mode of transfer of water is considered to be minor.
Second, since anion exchange membranes are positively charged, allowing for attachment and passage of negatively charged anions through their fine porous structures (which are filled with water and hydrated ions), induced movement of anions by the electric field also results in dragging of some of the water molecules from within the membrane pores. This coupled flow of water with ions under the influence of an electric field is defined as electro-osmosis. The same phenomenon occurs in negatively charged cation exchange membranes, which allow the passage of positively charged cations along with water through their pore structure. This phenomenon can be seen in electrodialysis experiments, in which water flow into the concentrate compartments and out of the diluate compartments is observed when the exit lines of both these compartments are closed and entry lines are monitored by such means as observing water level changes in input lines that were at the same level before application of the electric field. The rate of electro-osmotic flow can be mathematically described through the equation: Qe=Ke*E*A, where the flow, “Qe”, in m3/sec is governed by coefficient of electro-osmotic conductivity “Ke” in m2/volt*sec of the membrane, voltage gradient across the membrane “E” in volt/m and the area of the membrane “A” in m2. As a result, the amount of electro-osmotic flow per unit area is governed by coefficient of electro-osmotic conductivity which is a function of the type and structure of the membrane under consideration as well as solution concentration and the voltage gradient. For a given membrane, the higher the voltage step used across the membrane, the higher the electro-osmotic flow rate per unit area will be. Electro-osmotic flow could be reduced or stopped by applying a hydrostatic pressure to the other side of the membrane. The amount of pressure required is a function of the structure of the membrane as well as solution concentration, voltage gradient and coefficient of electro-osmotic conductivity.
The third mode of water movement in electrodialysis devices is through the process of osmosis. As the water in the diluate compartment becomes more dilute and as the water in the concentrate compartment becomes more concentrated, the concentration difference between the two leads to mobilization of osmotic pressure causing the flow of water from the diluate side to the concentrate side, while the larger ions are blocked. This process is independent of the electric field moving the ions. Osmotic pressure can be viewed as a compressive pressure imposed on the diluate side of the membrane to push the water molecules from the diluate side into the concentrate side. This convention of defining the direction of the osmotic pressure is used in this document. Osmotic pressure, which is governed by the ratio of solute particles to solvent particles in a solution, can be calculated for each solution using the Van't Hoff formula: π=cRT, in which osmotic pressure π is in Bars (kg/cm2), c is molar concentration of the solute in mol/liter, R is the gas constant equal to 0.082 (liter*Bar)/(degree*mol) and T is the temperature in degrees Kelvin. The difference in the osmotic pressure calculated for the dilute and the concentrated solutions is the osmotic pressure imposed on the membrane from the diluate side towards the concentrate side of the membrane. This pressure results in flow of water through the membrane, which can be stopped if the hydrostatic pressure on the concentrate side equals it. Indeed, if the pressure applied to the semi-permeable membrane between the two solutions, from the concentrate side, exceeds the osmotic pressure imposed on the membrane from the diluate side, then pure water will flow from the concentrate side to the diluate side. This is called reverse osmosis and is often used as a desalination technique.
The osmotic flow through a semi-permeable membrane can be calculated using Darcy's law Qh=Kh*I*A in which “Qh” is the hydraulic flow in m3/sec, “Kb” is the hydraulic conductivity of the semi-permeable membrane, “I” is hydraulic gradient in m/m (which is the hydraulic head created by osmosis divided by membrane thickness), and “A” is the membrane area in m2. This process is different from diffusion, which is defined as spreading of particles, and more specifically where there is either no membrane between mixing substances or the membrane between the two liquids has high conductivity for water as well as the dissolved ions. Although a theoretical discussion is not presented, it is noted that ion selective membranes used in electrodialysis equipment are semi-permeable membranes, allowing for osmosis and preventing diffusion. Passage of ions through ion selective membranes is only possible under the influence of an electric field or under pressure, provided electro-neutrality is maintained. That is, if a certain amount of charge is transferred from one liquid compartment to another through an ion selective membrane, there must be some means of neutralizing the remaining solution such as removing an equal amount of oppositely charged ions. Otherwise the resulting voltage buildup will prevent any further movement of ions.
Current efficiency is a measure of how effectively ions are transported across the ion exchange membranes for a given applied current. This means that when a given current “I” in Amperes passes through a diluate compartment for a given time “t” in seconds, the current efficiency could be defined as the ratio I*t to the charge transferred from the output diluate volume to the concentrate. As an example, if a current of 1.0 Amperes passes for period on 100 seconds between the electrodes of an electrodialysis cell, and if during the same period an equivalent of 80 Coulombs of charge is transferred from a diluate compartment to the two adjacent concentrate compartments, then the current efficiency is 80%. Typically current efficiencies of >80% are desirable in commercial electrodialysis operations to minimize energy operating costs. Low current efficiencies can be an indicator of water splitting in the diluate or concentrate streams, shunt currents between the electrodes, the occurrence of back-diffusion of ions from the concentrate to the diluate, or (as has been observed by this inventor) it could be caused by osmotic flow of diluate into the concentrate that reduces the output diluate volume. In typical electrodialysis devices, the speed by which the diluate flow is drawn from the diluate compartment can be increased in order to reduce the total osmotic flow. This requires faster removal of ions from the diluate compartment so that the desired product is formed faster and is also drawn out faster before much of it moves to the concentrate compartments. Within practical limits set by applicable voltages, currents and other limitations, the use of higher voltages to achieve faster desalination of the diluate and faster removal of the product is recognized as a method of improving the current efficiency.
Further optimization of current efficiency can also be achieved by control of the concentration of the solution in the concentrate compartments, by increasing the flow speed into these compartments, or by rapid displacement of these solutions through faster inflow of the input solutions to these compartments. That is, to limit osmotic flow between the diluate and the concentrate compartments, the process rate is increased by effecting faster ion transfer between compartments and higher fluid flow rates in the compartments. Since the rate of ion removal for a given concentration of inputs and outputs (the concentrate reject and the diluted solutions) is governed by Ohm's law and is therefore proportional to the intensity of the electric field (which is a function of the applied voltage between the electrodes), and since the amount of energy used to transfer a given amount of ions between compartments is also proportional to the voltage step applied to each compartment between the electrodes, the faster rate of production in these devices is achieved at the cost of higher energy use. As a result, there is a need to reduce the energy consumption per unit volume production in such devices by reducing the transfer rate of product water from the diluate to concentrate compartments.
Available literature regarding the current efficiency of electrodialysis equipment typically does not relate improved current efficiency to the application of pressure to the diluate/concentrate compartments. Rather, discussions range from claims that current efficiency is a function of feed concentration, to viewing current efficiency as a phenomenon affected by water splitting, deficient membrane ion selectivity, water transfer by osmosis/ion hydration, shunt currents, and back diffusion of ions from the concentrate to the diluate compartment. U.S. Patent App. Pub. No. 2011/0042219 to Wie et al., discloses the application of differential pressure to the electrodialysis unit input lines “to ensure minimal back diffusion” (paragraph [0025]), but it is not disclosed what this means exactly, and which line should be higher or lower in pressure or what pressures are needed. Further, many manufacturers recommend so-called zero trans-membrane pressure levels, which means that the hydrostatic pressure on both sides of these membranes are recommended to be the same. This is specified to prevent damage and puncture of the membranes. As such, other membrane manufacturers specify a maximum allowable pressure to prevent bursting of their membranes. U.S. Pat. No. 8,101,058 to Liang et al. discloses the use of a “pressure vessel” for raising the internal pressure of the device, but notes only that this can reduce the pressure difference between the interior and the exterior of the device, which can reduce manufacturing costs or simplify construction.
While the above-mentioned electrodialysis methods and devices may be useful for their intended purposes, there currently is no device or method for improving the current efficiency of electrodialysis systems by pressurizing the concentrate compartment as compared to the diluate compartment. It would thus be beneficial to provide a desalination device that can improve current efficiency in this manner. It would also be advantageous to provide an apparatus and method which allows for operation of electrodialysis or electrodeionization devices using lower voltages across the two electrodes and the consequent lowering of the voltage step for each cell compartment. It would also be beneficial to provide an electrodialysis device and method which reduces the energy consumption per unit volume of the product.